Various processes for producing cement clinker are known in the art. These processes generally reside in obtaining a clinker with predetermined properties by reacting calcium carbonate with alumina and silicon dioxide at 1350.degree.-1500.degree. C., with subsequent cooling of the resultant clinker.
Cement clinker is mainly produced by two methods: wet and dry, the difference between the methods residing in the preparation of a raw mix. In case clinker is produced by the wet method, a raw slurry is prepared, with subsequent roasting thereof in rotary kilns. With the dry method, a dry grinding of raw components is effected to prepare a raw meal which is subjected to a preliminary heat treatment with exhaust kiln gases in an extrakiln heat exchanger before transfer to the rotary kiln.
A modification of the dry method for producing cement clinker is a process involving the preparation of a raw meal, pelletization thereof and subsequent roasting of the resultant pellets. The roasting may be effected in rotary kilns having conveyor-type calcinators, or in a dense layer in shaft furnaces or on a sintering grate with the addition of a solid fuel to the raw meal.
For providing a required temperature in the furnace units, a gaseous, liquid or solid fuel is burned. (cf. E. I. Khodorov, Furnaces for Cement Industry (in Russian), M., Gosstroyizdat Publishing House, 1968).
The production of clinker is a rather energy-consuming process due to large fuel consumption for roasting raw materials.
At present a wet method for producing cement clinker is known which comprises grinding together raw components and a solution of calcium chloride added in an amount of 10 to 20% by weight of decarbonized batch, with subsequent roasting of the resultant batch in a rotary kiln. A salt melt based on calcium chloride is formed during roasting which ensures the completion of clinker-forming reactions at 1000.degree.-1200.degree. C. and preparation of a clinker with high grindability, whereas in roasting the raw slurry without calcium chloride this temperature is within the range from 1350.degree. to 1500.degree. C. (cf. USSR Inventor's Certificate No. 326152, 1965). Lowering the roasting temperature and heat consumption and improving the grindability of the resultant clinker enable a substantial reduction of energy consumption for the process. Residual content of chloride compounds in the resultant clinker exceeds acceptable limits from the point of view of ensuring desired construction and technical properties of cement. Accordingly, the clinker is subjected to a dechlorination before cooling by reacting it with a steam and air mixture at 900.degree.-1100.degree. C. The dechlorination resides in a thermal hydrolysis of unbonded calcium chloride with water vapour in accordance with the following reaction: EQU CaCl.sub.2 +H.sub.2 O=CaO+2HCl.uparw.
This is an endothermic reaction which occurs with the formation of gaseous hydrogen chloride. As a result of a contact between the resultant hydrogen chloride in the gaseous phase and carbonate compounds of the raw mix regeneration of calcium chloride occurs in accordance with the following reaction: EQU CaCO.sub.3 +2HCl=CaCl.sub.2 +H.sub.2 0.uparw.+CO.sub.2.
This is an exothermic reaction which is accompanied by a partial decarbonization of the raw mix and release of water vapour and carbon dioxide. The resultant calcium chloride participates in the clinker-forming process.
At present the above-described process is used only in rotary kilns exhibiting low thermal and production characteristics.
Known in the art is a process for producing cement clinker, comprising dry grinding raw components, mixing the resultant raw meal with a solid fuel, pelletizing the resultant batch with concurrent addition thereto of 10-12% by weight of calcium chloride, with subsequent heat treatment of the resultant raw mix in a dense layer in zones of drying, roasting by burning a fuel over the surface of the layer with drawing fuel combustion products therethrough at 900.degree.-1100.degree. C. with the removal of exhaust gases released from said layer, and cooling the resultant clinker (cf. USSR Inventor's Certificate No. 303315, 1967). The temperature conditions of the above-described process cannot ensure the binding of all calcium oxide into clinker materials in the part of the layer located on the side of admission thereto of products of combustion of the fuel burnt over the layer.
The content of 10-12% by weight of calcium chloride in the raw mix fed for roasting results, in case of an accelerated roasting, in a disruption of the correlation between the process temperature and quantity of liquid salt melt required for the clinker-forming reactions to occur. The consequence of this is lowering of the speed and completeness of combustion of the fuel which is present in the raw mix which causes a decrease in the vertical rate of sintering of the raw mix in the roasting zone and an increase in the heat losses due to a mechanical underburning of solid fuel, and in a worse quality of clinker.
Forming a dense layer of raw mix pellets with uniform content of calcium chloride over the whole thickness of the layer results during roasting in a substantial increase in the calcium chloride concentration in pellets of the part of the layer located on the side of removal of exhaust gases released therefrom. The latter circumstance is due to a partial absorption by the pellets of hydrogen chloride released during the dechlorination of clinker in the foregoing parts of the layer. The above reasons result in the formation of an excessive quantity of liquid phase in the pellets causing the formation of heavy aggregates, disruption of gas permeability of the layer which results in production of clinker of poor quality and in an inadequate intensity of the roasting process.